If you live in a hard water area, and you've got a stainless steel sink top, you should be familiar with the problems of trying to keep it free from limescale. I would guess that it's the possible build up of limescale on the stainless steel ball that would be the most likely problem. (It would cease to be shiny and thus far less attractive). Limescale might also build up in the piping, slowing the flow of water to a dribble.
Wilkinsons sell distilled water. (A web page I found quoted 49p per litre in 2007).
Chris



